Ice Dam Risk Calculator

How It Works

Ice dams form when heat escaping from the house melts snow on the upper roof, which then refreezes at the cold eave overhang, creating a dam that traps water behind it. This water backs up under shingles and leaks into the house. The three keys to prevention are: (1) adequate insulation (R-49+) to keep heat inside, (2) proper attic ventilation to keep the roof deck cold, and (3) air sealing to block warm air leaks into the attic. Recessed can lights, plumbing vents, and attic hatches are the worst air leakers. Ice and water shield membrane at the eaves provides a last-resort waterproof barrier. Heat cables (roof de-icing) treat the symptom, not the cause — they are expensive to operate and should be a last resort.